The present invention relates generally to an automatic transmission which includes a torque converter with a lock-up mechanism and is disposed for a speed ratio change in the power transmission path between the engine and the drive wheels of a vehicle. Furthermore, the present invention relates particularly to a control system that controls the engagement of the lock-up mechanism and the shift of the automatic transmission.
Automatic transmissions have been used in vehicles. Generally, an automatic transmission for use in a vehicle comprises a plurality of gear trains, which are disposed parallel with one another, a plurality of frictionally engaging elements such as friction clutches, which are used for selecting a certain gear train for power transmission from these gear trains, and a shift control valve, which controls the actuation of the frictionally engaging elements. For such an automatic transmission, for example, Japanese Laid-Open Patent Publication Nos. 11(1999)-108169 and 11(1999)-201270 disclose a shift control system.
In the shift control performed by the control system, the shift control valve functions to release a clutch which has been in engagement (hereinafter referred to as xe2x80x9coff-going clutchxe2x80x9d or xe2x80x9coff-going frictionally engaging elementxe2x80x9d) and to bring another clutch into engagement (hereinafter referred to as xe2x80x9con-coming clutchxe2x80x9d or xe2x80x9concoming frictionally engaging elementxe2x80x9d) for shifting the gear trains of the transmission, from the off-going speed ratio to the on-coming speed ratio. If the release of the off-going clutch and the engaging actuation of the on-coming clutch are not controlled appropriately, then a shift shock, an engine racing, etc. may happen, damaging the smooth shifting of the gear trains. To avoid such discomforts, it is important to control these clutches in a timely manner. Therefore, a various methods and systems have been proposed to appropriately control the release of the off-going clutch and the engagement of the on-coming clutch. For example, Japanese Laid-Open Patent Publication Nos. 7(1995)-71579 and 9(1997)-317874 disclose a control system that comprises a plurality of control valves and solenoid valves for the purpose of achieving a smooth shift operation.
In general, an automatic transmission comprises a torque converter, which is connected to the output shaft of the engine, and often, this torque converter is equipped with a lock-up mechanism. In this case, the control system comprises various valves that are to control the lock-up mechanism in addition to those valves that are used for the shift control of the transmission.
By the way, the number of the valves constituting the shift control system of an automatic transmission has increased corresponding to the increase of the number of the speed ratios available for the transmission, and the number of the valves has increased even more as the control system includes additionally not only an on-off solenoid valve but also a duty solenoid valve, a linear solenoid valve, etc. for a smooth shift operation. The same can be said of the number of the valves for the control of the lock-up mechanism. As the number of the valves necessary for the control system increases, there is a problem of the design of the system becoming complex and sizable.
It is an object of the present invention to provide a control system for a vehicular automatic transmission, which system has a relatively simple construction because of a valve that is used commonly for controlling the operation of a shift control valve and the operation of a lock-up control valve.
In order to achieve this objective, the present invention provides a control system for an automatic transmission for use in a vehicle, which transmission comprises a torque converter equipped with a lock-up mechanism (for example, the lock-up clutch 4 described in the following embodiment) and is disposed in the power transmission path between the engine and the driving wheels. The automatic transmission further comprises a plurality of frictionally engaging elements (for example, the FIRSTxcx9cFOURTH speed clutch 31xcx9c34 described in the following embodiment), which are actuated for engagement upon receiving a hydraulic pressure to establish selectively a plurality of speed ratios, and a shift from an off-going speed ratio to an on-coming speed ratio is executed by a controlled release of the hydraulic pressure from the frictionally engaging element used for the off-going speed ratio and by a controlled supply of the hydraulic pressure to the frictionally engaging element used for the on-coming speed ratio. For this transmission, the control system according to the present invention comprises an off-going pressure releasing valve (for example, the first and second off-going pressure releasing valves 70 and 80 described in the following embodiment), which releases the hydraulic pressure from the frictionally engaging element used for the off-going speed ratio during the shift from the off-going speed ratio to the on-coming speed ratio, a lock-up control valve, which controls the engagement of the lock-up mechanism, and an actuation control valve (for example, the linear solenoid valve 60 described in the following embodiment), which supplies a control pressure to both the off-going pressure releasing valve and the lock-up control valve and controls the operation of the off-going pressure releasing valve and the lock-up control valve.
For the lock-up mechanism to function appropriately, the engagement and disengagement of the lock-up mechanism must be always controlled in correspondence to the condition of the transmission while the vehicle is traveling. However, the shift control of the transmission is required only during a shift while the vehicle is traveling, so the shift control should take priority over the engagement control of the lock-up mechanism during the shift. The present invention takes notice of this point. The control system according to the present invention controls, by one actuation control valve, the operation of the off-going pressure releasing valve, which releases the hydraulic pressure from the frictionally engaging element used for the off-going speed ratio during the shift, and the operation of the lock-up control valve. The control system in this arrangement requires fewer actuation control valves than a prior-art system which controls the operation of the off-going pressure releasing valve and the operation of the lock-up control valve by separate actuation control valves. Thus, the control system according to the present invention can be realized in a simple construction and in a small size.
The lock-up control valve may comprise a lock-up shift valve, a lock-up control valve, a lock-up timing valve and a lock-up switch solenoid valve. In this case, the lock-up shift valve is actuated by a hydraulic pressure from the lock-up switch solenoid valve to switch a condition for engaging the lock-up mechanism and for releasing the lock-up mechanism. While the lock-up shift valve is set in the condition for engaging the lock-up mechanism, the supply of the control pressure from the actuation control valve to the off-going pressure releasing valve is restricted, but the control pressure is supplied to the lock-up control valve and the lock-up timing valve, so that the operation of the lock-up mechanism is controlled by the actuation control valve. On the other hand, while the lock-up shift valve is set in the condition for releasing the lock-up mechanism, the control pressure from the actuation control valve is supplied to the off-going pressure releasing valve, so that the release of the hydraulic pressure from the frictionally engaging element used for the off-going speed ratio is controlled by the actuation control valve.
With this construction, the operation of the lock-up shift valve is controlled by the lock-up switch solenoid valve to switch securely the condition of the lock-up mechanism for engagement and for release. Upon establishing the right condition, if the lock-up mechanism is in the engagement condition, then the lock-up engagement control is executed by the actuation control valve, or if the lock-up mechanism is in the release condition, then the control for releasing the hydraulic pressure from the frictionally engaging element used for the off-going speed ratio is executed for the shift, by the actuation control valve. In other words, only when a shift is executed, the lock-up release condition is established by the lock-up shift valve, and the off-going pressure releasing control is executed by the actuation control valve. In this way, the actuation control valve is used both for the lock-up engagement control and the off-going pressure releasing control.
Furthermore, while the lock-up mechanism is being engaged, the pressure output from the actuation control valve is supplied to both the lock-up control valve and the lock-up timing valve. In this condition, the engaging capacity of the lock-up mechanism is adjustable continuously and smoothly from a small lock-up engagement capacity to a maximum lock-up engagement capacity by one actuation control valve. Thus, the present invention can realize an improved fuel efficiency and controllability.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.